Continuous Positive Airways Pressure (CPAP) machines are well known in the art for use in the treatment of a number of respiratory conditions, such as sleep apnea and hypopnea, by supplying a continuous positive pressure to a patient's airway while the patient sleeps. A typical CPAP apparatus is programmed with a CPAP therapy pressure, and is able to maintain the set pressure (measured either at the mask or at a base unit) during the inhalation and exhalation phases of the breathing cycle.
In a variation of the basic CPAP machine, two air pressure levels are able to be programmed into the machine, an inspiratory positive airway pressure (IPAP), delivered during the inhalation phase of the breathing cycle, and a expiratory positive airway pressure (EPAP), delivered during the exhalation phase of the breathing cycle. Typically, the EPAP pressure is lower than the IPAP pressure, resulting in a pressure profile over time having a square wave shape, as shown in FIG. 2.
This arrangement tends to increase the patient's comfort while using the machine by providing the lowest possible EPAP pressure necessary to maintain airway patency, thereby reducing the work required for the patient to exhale. Typically, the machine is able to sense the patient's breathing rhythm, and is able to detect the transitions between the inhalation and exhalation phases of the breathing cycle, such that the proper pressure can be delivered to the patient. This version of the CPAP machine is often referred to as a Bi-Level PAP or Bi-PAP machine.
One problem with the typical Bi-Level PAP machine is that the transitions between the IPAP and EPAP pressures can be abrupt, causing discomfort to the patient, especially when transitioning from the EPAP to the IPAP pressure. Attempts have been made to solve this problem by providing a means for adjusting the transitions between the EPAP and IPAP pressures. As an example, U.S. Pat. No. 5,865,173 provides a means for the therapist to select a rounding factor that is applied to the pressure waveform to smooth the transitions between the two pressures. Various degrees of rounding may be selected to maximize the patient's comfort. Likewise, U.S. Pat. No. 7,296,573 provides a set of pre-defined pressure contours which are stored in the unit's memory and which may be selected by the therapist to maximize both the patient's comfort and also the effectiveness of the therapy. Both the '173 patent and the '573 patent have limitations in their ability to customize the shape of the pressure curve. Therefore, it would be desirable to provide a method of selecting the waveform that provides increased flexibility to further maximize both the patient's comfort and the effectiveness of the therapy delivered to the patient.